Spice level implementation of Barrel shifter using 2:1 Mux

1. A
PROJECT REPORT
ON
“4-BIT BARREL SHIFTER”
Nikhil Umesh Dantkale
Class-TE Div-N Roll No.-23
DEPARTMENT OF ELECTRONICS ENGINEERING
VISHWAKARMA INSTITUTE OF TECHNOLOGY
(An Autonomous Institute, Affiliated to Pune University)
Academic Year 2012-13

2. CERTIFICATE
VISHWAKARMA INSTITUTE OF TECHNOLOGY
Department of Electronics Engineering
This is to certify that Nikhil Umesh Dantkale (Roll No:N-23) has submitted a
project on the topic “ 4-BIT BARREL SHIFTER ”in Mixed Signal VLSI Design LAB
(VLSI Honor) for the academic year 2012-13.
Prof. A.V. Pedgaonkar Prof. A.M.Chopde
Assistant Professor Head of the Department
Electronics Engineering

3. ACKNOWLEDGEMENT
It is matter of great pleasure for me to submit this project report on
“4-BIT BARREL SHIFTER”, as a part of curriculum for award of
“Bachelor of Engineering in Electronics & Telecommunication”.
I am thankful to my project guide Prof. A. V. PEDGAONKAR
Assistant Professor in Electronics Engineering Department for her constant
encouragement and able guidance.
I am also thankful to Prof. A. M. Chopde, Head of Electronics
Engineering Department for his valuable support. I take this opportunity to
express my deep sense of gratitude towards those, who have helped me in
various ways, for preparing my project. At the last but not least, I am thankful
to my parent, who had encouraged & inspired me with their blessings.
(Name of Student)
Nikhil U. Dantkale

4. TABLE OF CONTENT
Sr
No.
Content Page No
1. Introduction 6
2. Implementation of Barrel Shifter
using 2:1 Multiplexer
7
3. Transistor Level Diagram of 2:1
Multiplexer
8
4. Code For 4-Bit Barrel Shifter using
2:1 Multiplexer
9
5. OUTPUT WAVEFORMS OF
BARREL SHIFTER
11
6. Advantages of Barrel shifter 12
7. APPLICATIONS OF BARREL
SHIFTER
13

5. ABSTRACT
A barrel shifter is a combinational logic circuit with n data inputs, n data
outputs, and a set of control inputs that specify how to shift the data between
input and output.
In this project we have implemented 4-Bit Barrel Shifter which has 4 data inputs
D0,D1,D2,D3 and 4 data outputs Y0,Y1,Y2,Y3 using VLSI Technology. Three modules
have been designed which consist of four 2:1 Multiplexer in each module. Each module has
one select line. Select lines of first and third module determines how many bits to shift.
Select line of second module specifies shifting of data either to left or right. Third module
provides you your desired output.
We have implemented 2:1 mux using transmission gates which requires 6
transistors. Thus we have implemented 4-Bit Barrel Shifter using 72 transistors.
We designed a 4 bit Barrel Shifter that shifts data from an input to an output
depending on the combination of two select lines. Our design of the Barrel shifter allows for
an increase in the number of input bits without having to modify the existing design.

6. INTRODUCTION
A barrel shifter is a digital circuit that can shift a data word by a specified number
of bits. It can be implemented as a sequence of multiplexers (mux.), and in such an
implementation the output of one mux is connected to the input of the next mux in a way that
depends on the shift distance.
A barrel shifter that is part of a microprocessor CPU can typically specify the
direction of shift (left or right), the type of shift (circular, arithmetic, or logical), and the
amount of shift (typically 0 to n-1 bits, but sometimes 1 to n bits), where 'n' is data inputs and
data outputs of barrel shifter.
Barrel shifters are often required for performing data shifting and rotation in many
key computer operations from address decoding to computer arithmetic. Barrel shifter
circuits are essential elements in the design of data paths for DSP applications. A significant
reduction in area and power required by the barrel shifter circuit is achieved by implementing
rightward operations as operations in leftward direction. A significant reduction in delay is
possible by reducing the length of critical path.

7. Implementation of Barrel Shifter using 2:1 Multiplexer

8. Transistor Level Diagram of 2:1 Multiplexer
Fig. Transistor Level Diagram of 2:1 Multiplexer using Transmission Gate

9. Code For 4-Bit Barrel Shifter using 2:1 Multiplexer
*barrel shifter
.subckt two 1 4 5 6
.model p pmos
.model n nmos
m1 3 1 0 0 n
m2 3 1 2 2 p
m3 4 3 6 0 n
m4 6 1 4 2 p
m5 5 1 6 0 n
m6 6 3 5 2 p
vdd 2 0 dc 5v
.ends
X1 6 1 3 7 two
X2 6 2 4 8 two
X3 6 3 1 9 two
X4 6 4 2 10 two
X5 15 8 10 11 two
X6 15 9 7 12 two
X7 15 10 8 13 two

10. X8 15 7 9 14 two
X9 5 7 11 16 two
X10 5 8 12 17 two
X11 5 9 13 18 two
X12 5 10 14 19 two
v1 1 0 pulse(0 0 0ns 0ns 0ns 1000ns 1000ns)
v2 2 0 pulse(0 5 0ns 0ns 0ns 1000ns 1000ns)
v3 3 0 pulse(0 5 0ns 0ns 0ns 1000ns 1000ns)
v4 4 0 pulse(0 0 0ns 0ns 0ns 1000ns 1000ns)
v5 5 0 pulse(0 5 100ns 0ns 0ns 125ns 250ns)
v6 6 0 pulse(0 5 200ns 0ns 0ns 250ns 500ns)
v7 15 0 pulse(0 5 0ns 0ns 0ns 500ns 1000ns)
.tran 0.01ns 1000ns
.plot v(1) v(2) v(3) v(4) v(5) v(6) v(15) v(16) v(17) v(18) v(19)
.end

11. OUTPUT WAVEFORMS OF BARREL SHIFTER

12. ADVANTAGES
1. Using Barrel shifter input word is either shifted left or right or remains unchanged as
per select line at a time, whereas in shift registers, data is shifted by one bit on the
rising edge of clock.
2. The propagation delay of Barrel Shifter is theoretically constant and independent of
the shift value or shifter size.
3. Barrel shifter is appropriate for smaller shifts. For larger shift values, the logarithmic
shifter becomes more effective.

13. APPLICATIONS OF BARREL SHIFTER
1. Several microprocessors incorporate it as a part of their ALU to provide fast shift
operations.
2. Barrel shifter circuits are essential elements in the design of data paths for DSP
applications.
3. Barrel shifters are often required for performing data shifting and rotation in many
key computer operations from address decoding to computer arithmetic.
4. It is used extensively in floating-point units, scalars, and multiplications by constant
numbers.